The invention is directed towards a method of screening semiconductor materials for use as chemical sensors, and a method to determine the conductivity type of semiconductors.
The need for chemical sensors capable of detecting gases such as nitrogen oxides, hydrocarbons, carbon monoxide and oxygen at the ppm to the percent level is longstanding. These sensors can be used in many applications, such as automotive exhaust sensing, or detection of toxic atmospheres. Taguchi, or semiconducting resistance-type, sensors are seen as the most likely candidates for these types of applications. Of particular value are semiconducting metal oxides whose electrical resistance varies with the composition of the surrounding gaseous atmosphere. To date only a very limited number of semiconducting compositions have been examined. Thus there is a further need to develop a rapid parallel screening technique for candidate materials.
In semiconducting materials, conduction of electricity is explained in terms of majority and minority carriers of electric charge. In n-type semiconductors, electrons are the majority carriers and holes, i.e. the spaces left by electrons, are the minority carriers. In p-type semiconductors the opposite is true; the holes are the majority carriers and the electrons are the minority carriers.
Previously researchers have used the xe2x80x9chot probexe2x80x9d technique to measure p- or n-type. Another test for p- or n-type involves forming a contact diode with a wafer by means of a probe. The direction of current flow, either d.c. or a.c., through the diode indicates conduction type. Both of these methods are slow and use expensive, bulky equipment. They are also not suitable for rapid, parallel methods of screening for large numbers of materials.
Disclosed is a method for determining the change in resistance of a semiconducting material in response to exposure to a sample gas by: a) applying a voltage bias across the semiconducting material; b) measuring the difference between the temperature of the material as exposed to said sample gas and the temperature of the material as exposed to a reference gas; and c) relating the measured difference in temperature to a change in resistance. The voltage bias is preferably about 0.5 V to about 200 V, and the difference in the temperature is preferably measured with an infrared thermographic measuring system. The semiconducting material is preferably a metal oxide deposited on a solid substrate.
Also disclosed is a method for parallel screening of semiconducting materials for suitability as chemical sensing materials by determining the resistance change in a plurality of semiconducting materials in response to a sample gas by: a) applying a voltage bias across each semiconducting material; b) simultaneously measuring the difference between the temperature of each material as exposed to said the sample gas and the temperature of each material as exposed to a reference gas; and c) relating the measured difference in temperature for each material to a change in resistance of that material.
Also disclosed is a method for the parallel screening of a plurality of semiconducting materials for suitability as chemical sensing materials by:
a) applying a voltage bias across each semiconducting material;
b) simultaneously measuring the difference between the temperature of each material as exposed to a sample gas and the temperature of each material as exposed to a reference gas; and
c) comparing the measured difference in temperature exhibited by a first material to the measured difference in temperature exhibited by a second material.
Also disclosed is a method for determining the conductivity-type of a semiconducting material by: a) applying a voltage bias across the semiconducting material; b) measuring the difference between the temperature of the material as exposed to a sample gas and the temperature of the material as exposed to a reference gas; and c) relating the measured difference in temperature to a conductivity type.